Volume control for sound reproduction systems



Aug 8, 1950 P. K. BUYS ETAL 2,517,629

VOLUME CONTROL FOR SOUND REPRODUCTION SYSTEMS Filed March 26, 1948 PIEIEBZZA/JJ Ul/J PIEZZIBIWEH/f VA/UZMK .INVENTORS A GENI Patented Aug. 8, 1950 VOLUME CONTROL FOR SOUND REPRODUCTION SYSTEMS Pieter Klaas Buys and Pieter Frederik van Eldik, Eindhoven, Netherlands, assignors to Hartford National Bank and Tru Conn., as trustee st Company, Hartford,

Application March 26, 1948, Serial No. 17,224 In the Netherlands April 29, 1947 6 Claims. 1

"The invention relates to an arrangement for automatic adaptation of the amplication factor of a low-frequency amplifier comprising reproducing apparatus to the sound level of the surroundings of this reproducing apparatus. The arrangement is more particularly intended for use as a loudspeaker system on railway yards, where the intensity of sound energy produced by the station loudspeaker with respect to that of the` surroundings is required to be a constant number of decibels, for example 40 decibels, higher, since in this case the intelligibiiity of the sound emitted by the loudspeaker will always be the same.

A known arrangement for this purpose comprises a microphone, an amplier and a loudspeaker, while in the surroundings of the loudspeaker provision is made of an auxiliary microphone from which a control-voltage is taken which controls the ampliiication factor of the said ampliiier. Such an arrangement entails the diiculty that the auxiliary microphone not only receives the sound of the surroundings but also part of the sound energy of the loudspeaker, so that there is a risk of acoustic back-coupling. In the known arrangement, efforts have been made to solve this problem by deriving the control voltage with the aid of which the ampliication factor of the amplifier is controlled from the diiierence between a voltage derived, by rectication, from the auxiliary microphone and a voltage which is proportional to the amplitude of the current ilowing through the microphone in such manner that this diiierence voltage is only a measure of the sound level of the surroundings. This method ensures that the sound energy produced by the loudspeaker exceeds by approximately decibels that of the surroundings. When aiming at a higher value of this energy ratio, the diniculty is experienced that the two components, the diierence between which must yield the control-voltage, have a very high value with respect to this control voltage, so that very small phase-displacements between these two components, provoked, for example, by a variation of the acoustic spacing between the loudspeaker and the auxiliary microphone, may cause acoustic back-coupling.

The invention permits the enrgy ratio between the sound produced by the loudspeaker and that produced by the surroundings to acquire an arbitrary value, since only during the time in which no acoustic energy is emitted by the loudspeaker will a control voltage which controls the ampliiication factor of the amplifier be derived from the auxiliary microphone. From the instant the loudspeaker emits acoustic energy, this controlvoltage acquires a value independent of the Voltage set up at the auxiliary microphone.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanyingr drawing, given by way of example, in which Fig. l represents in block form the system according to the invention and Figs. 2 and 3 represent embodiments of the invention.

In Fig. l the reference numerals l and 2 designate two amplication stages with the aid of which the electrical oscillations derived from a microphone 3 are ampliiied and, by means of reproducing apparatus (for the sake of simplicity represented by a loudspeaker 4) converted into acoustic oscillations. The loudspeaker 4 is housed in a space (for example in a railway yard) where at the same time other sound sources 5 (for eX- ample moving trains) are operative. The sound energy produced by the loudspeaker 4 and the sources 5 is picked up by an auxiliary microphone 6 from which, with the aid of a circuit l, a control-voltage is derived which controls, through a lead 8, the ampliication of the amplifier 2.

According to the invention the circuit 'l is controlled in such manner by an alternating voltage supplied through a lead 9 that only if this alternating voltage is equal to zero and, consequently, the energy emitted by the loudspeaker 4 is equal to zero, will the control voltage produced in the lead 8 depend upon the voltage produced by the auxiliary microphone ii. From the instant the alternating voltage of the lead 9 is not equal to zero, for example due to speaking into the microphone 3, the control-voltage produced in the lead 8 is independent of the Voltage set up at the auxiliary microphone.

Figs. 2 and 3 show suitable examples of such a circuit l. They are based on the fact that a condenser I i, having a capacity of a few ef., through which a voltage controlling the control voltage is produced, is rapidly charged to a voltage corresponding to the sound level of the surroundings and fed to the terminals l0 during the time the alternating voltage set up at the terminals 9 is equal to zero, whereas this condenser is very slowly discharged during the time this voltage is not equal to Zero. In order to ensure a very slow discharge o1" the condenser H, the circuits comprise a series-circuit which is controlled by a voltage supplied through the lead l!) to the auxiliary microphone S, this series-circuit comprising the discharge tubes l2 and i3 which pass current in the same direction and a source of supply voltage I4. One terminal of the condenser I I is connected to a point I5 having a potential dened with respect to the source of supply voltage Iii, whereas the other terminal is connected to a point of the said series-circuit located between the two discha-rge tubes, the two ldischarge tubes I2 and I3 being conductive only if the alternating voltage set up at the terminals 9 is equal to Zero. Consequently, the condenser l I can only be discharged through insulating resistances having a total value of., say i000 MQ, so that a very long discharge time constant of several thousands of seconds may be achieved.

According to Fig. 2 the oscillations fed to the terminals 9 are rectified with the aid of a rectifier I6 and smoothing filter Il, whereupon this rectified voltage is sufficient to open the normally nonconductive, gas lled discharge tube I8 which isv :fled` by a source of alternating voltage I9. The alternating voltage produced in the output transformer E@ of the tube I3 is rectiiiecl with the aid of the two rectifying circuits ZI and 22, the output voltages of which suffice to cut off the two tubes I2 and I3. The time constant of the circuit IE, Il is of the order of magnitude of several milliseconds and consequently small enough to prevent any undue acoustic back-coupling phenomena. The discharge time constant of the circuits 25 and 22 is, for example, of the order of magnitude oi a few tenths of a second.

If no voltage is produced across the lead 9 the tube I8 is cut oii, so that the tubes I2 and I3 are conductive. In the grid circuit of the tube I2 the voltage across a smoothing filter 2 3 is operative. This is achieved by rectiiying the voltage fed to the terminals I0 with the aid of a rectifier 213, owing to which the conductivity of the tube I2 is controlled in accordance with the voltage across the terminals I0 i. e. in accordance with the sound energy of the surroundings. Hence the condenser II acquires a potential which is determined by the ratio between the resistance of the tube I2 comprising a cathode resistance 25 and the sum of the resistances of the tube I2 and I3, the time constant for charging the condenser II being determined by the product of the. value of this condenser and the parallel-resistance of the aforesaid resistance and having. for instance, a value of 50 insecs. The voltage produced across the condenser Il isv ainpliiied with the aid of the tube 26, after which the control-voltage is taken, through the filter 2l having a time constant of some few tenths of a second and eiiecting a gradual variation of the ampliiication factor, from the terminal 8.

In the circuit-arrangement shown in Fig. 3 the oscillations supplied through the terminals 9 are ampliiied and rectified with the aid of the tube I6 comprising a high anoderesistance 29 and a decoupling condenser I'I. The voltage produced across the resistance is operative in the gridcircuits of tubes l2, and I3 and renders these tubes conductive only when the alternating volt age set up Iat the terminals 9 is equal to zero. The time required for charging the condenser II (for example of 0.1 uf.) is only 1 msec., since the internal resistance of the tube I6, owing to negative back-coupling, is, for example, only l0 kc, the discharging time of this condenser being a few tenths of a second.

In the series-connection of the discharge tubes I2 and I3, a voltage supplied to the terminals HI.v through 'an input transformer is operative Y the microphone i.

during the time the tubes I2 and I3 are conductive. This causes the condenser to be charged through the tube I2 to a value corresponding to the amplitude of the voltage fed to the terminals I0, the tube I3 l'iavingy an anode resistance of, say, 10 kn, serving as a discharge circuit. The charging time of the condenser II is, for example, 50 msec.

The negative control--vcltage taken from the terminal S controls, in a known manner, the amplication factor of the amplification stage 2. It affects, for example, the grid adjustment of one or more of the discharge tubes included in the ampliiication stage 2.

In addition to the control as described it is desirable to provide in the said loudspeaker system a second control which ensures that the amplitude of the input voltage of the amplifier 2 has a substantially constant value, so that the sound energy emitted by the loudspeaker Il is independent of the distance of the speaker from` This second control may be provided in the form of a well-known automatic gain control (designated in Fig. l).

The invention is, however, not considered toj be limited to such a loudspeaker system and for example, also applies to an arrangement in which the microphone 3 comprising the amplifier I and automatic gain control 30 shown in Fig. l is replaced, for example, by `a gramophoneor a radio-receiving set.

What we claim is:

l. A sound reproduction system of the type adapted to maintain an acoustic energy output at a xed level above the ambient noise level, comprising in combination: a source of audio frequency electrical impulses including a microphone and an ampliiier; a variable gain ampli fier connected to said source; at least one loudspeaker connected to the output of said variable gain amplifier; a second microphone located in the acoustic range of said loudspeaker; a condenser; means including a pair of series-con-v nected normally-conductive electron discharge tubes connected to charge said condenser; means coupling said microphone to said irst mentioned means whereby said condenser may charge in accordance with the ambient noise level; means including a normally nonconductive thyratron connected between said source and said iirst mentioned means, and connected to apply a cut ori bias to said tubes whenever said thyratron is triggered by said source; and means including a third electron discharge tube connected to amplify the voltage on said condenser and couple the resulting control voltage to said vari-able gain ainplier whereby the gain of said amplifier is caused to vary in proportion to the ambient noise level Whenever said loudspeaker is de-energized. i

2. A sound reproduction system of the type adapted to maintain an acoustic energy output at a fixed levelA above the ambient noise level, comprising in combination: a source of audio frequency electrical impulses including a microphone and an ampliner; a variable gain amplifier connected to said source; at least one loudspeaker connected to the output o1 said variable gain amplier; a second microphone located in the acoustic range of said loudspeaker; a condenser; means including a pair of series-connected normally-conductive electron discharge tubes connected to charge said condenser; means coupling said microphone to said first mentioned means' whereby said condenser may charge in accordance with the ambient noise level; means including a vacuum electron discharge tube connected between said source and said iirst mentioned means; a resistor' in the anode circuit of said tube; means to couple a negative voltage from said resistor to said series-connected discharge tubes to drive said tubes to cut-off whenever acoustic energy is applied to said nrst mentioned microphone; and means including a fourth elec-- tron discharge tube connected to amplify the voltage on said condenser and couplethe resulting control voltage to said variable gain amplifier whereby the gain of said amplifier is caused to vary in proportion to the ambient noise level whenever said loudspeaker is dfi-energized.

3. A sound reproduction system, comprising a source of audio frequency impulses, a variable gain amplifier coupled to said source, sound reproducing means coupled to the output oi said amplifier and having a given acoustic range, ambient sound pickup means located in said acoustic range, a control voltage producing device coupled to said sound pickup means, means to apply the output of said control voltage producing device to said amplifier to control the gain thereof in accordance with the ambient sound level in said acoustic range, and means comprising an electron discharge tube having an input circuit coupled to said source of audio frequency impulses and an output circuit coupled to said control voltage producing device to render the output of said control voltage producing device insensitive to ambient sound variations when said audio frequency impulses are applied to said amplifier.

4. A sound reproduction system, comprising a source of audio frequency impulses, a variable gain amplifier coupled to said source, sound reproducing means coupled to the output of said amplifier and having a given acoustic range, ambient sound pickup means located in said acoustic range, a control voltage producing device coupled to said sound pickup means, said control voltage producing device comprising a capacitor, means to charge said capacitor proportional to the ambient sound level in said acoustic range and means to derive an output control voltage from said capacitor, means to apply said output control voltage to said amplifier to control the gain thereof in accordance with the ambient sound level in said acoustic range, and means comprising an electron discharge tube having an input circuit coupled to said source of audio frequency impulses and an output circuit coupled to said control voltage producing device to prevent charging of said capacitor when said audio frequency impulses are applied to said amplifier.

5. A sound reproduction system, comprising a source of audio frequency impulses, a variable gain amplier coupled to said source, sound reproducing means coupled to the output of Said amplifier and having a given acoustic range, ambient sound pickup means located in said acoustic range, a control voltage producing device, said control voltage producing device comprising a capacitor, irst and second serially-connected normally conductive electron discharge tubes coupled to said capacitor, means to couple said first and second discharge tubes to said sound pickup means thereby to charge said capacitor proportional to the ambient sound level in said acoustic range and means to derive an output control voltage from said capacitor, means to apply said output control voltage to said amplifier to control the gain thereof in accordance with the ambient sound level in said acoustic range, and means comprising a third electron discharge tube having an input circuit coupled to said source of audio frequency impulses and an output circuit coupled to said first and second discharge tubes to render said rst and second discharge tubes non-conductive when said audio frequency impulses are applied to said amplifier.

6. A sound reproduction system, comprising a source of audio frequency impulses, a variable gain amplifier, an amplification stage having a substantially constant gain intercoupling said source of audio frequency impulses and said variable gain amplier, sound reproducing means coupled to the output of said amplier and having a given acoustic range, ambient sound pickup means located in said acoustic range, a control voltage producing device coupled to said sound pickup means, means to apply the output of said control voltage producing device to said ampliiier to control the gain thereof in accordance with the ambient sound level in said acoustic range, and means comprising an electron discharge tube having an input circuit coupled to said source of audio frequency impulses and an output circuit coupled to said control voltage producing device to render the output of said control voltage producing device insensitive to ambient sound variations when audio frequency impulses are applied to said amplifier.

PIETER KLAAS BUYS. PIETER FREDERIK VAN ELDIK.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,338,551 Stanlio Jan. 4, 1944 2,382,848 Baumgartner Aug. 14, 1945 2,392,218 Anderson Jan. 1, 1946 2,420,933 Crawford May 20, 1947 2,457,712 Olson et al Dec. 28, 1948 2,462,532 Morris Feb. 22, 1949 

